The space overhead comes about because of the definition of new forwarding methods. The increased class footprint might result in extra runtime cost of classloading and also some minor impact on memory addressing and cache misses because the class file's memory footprint is slightly larger than before. (thanks to Josh for the explanation)

Extensive use of forwarding methods may also have a noticeable effect on stack usage patterns in multithreaded environments (like a web server). Without forwarding, the stack goes wind-unwind-wind-unwind-wind-unwind as each method returns before you make the next call. With forwarding, the stack goes wind-wind-wind-unwind-unwind-unwind. This may push up the peak memory usage per thread.

However, for all practical purposes all of the above effects are minor compared to the not-so-minor gains in encapsulation.